INT Dome

General Description

The dome is a steel structure consisting of pre-fabricated elements which are welded to form the hemispherical dome. The dome roller bogies are enclosed on the outside by a removable skirt including a weather seal and inside by another removable skirt which incorporates a nylon brush seal. The dome rotates on a flat top rail which is mounted to give a truly circular and horizontal rolling surface. The axes of the 46 rollers are arranged such that they slope inwards to intersect at a point which coincides with the centre of rotation. This ensures that each roller is guided on a circular path. Three of the rollers are connected to drive systems to provide dome rotation. These are equally spaced around the dome. The dome position is encoded by a 10 bit absolute T + R encoder giving positional accuracy of ~ +/- 2 inches of travel. The encoder is driven by an endless chain which is a fixed to the dome circumference.

The top and bottom shutters are of the 'up and over' type. Both are driven via motors and gearboxes combined with a drum which carries the steel driving cables. The motors and cable drums ride with the shutters. Limit switches are operated by both shutters to enable and disable the main drive or micro drives of each shutter.

Dome Control

The dome can be rotated locally from the BRITTAIN dome/shutter control cabinet (which rides with the dome) when the keyswitch is put to LOCAL or in either engineering or computer mode with the keyswitch set to REMOTE. Both engineering and computer modes communicate with the dome electronics using TEM-L equipment. In engineering mode, push buttons on the engineering desk control the dome direction; the speed being set using thumb wheel switches.

In computer mode, TCS control for dome direction and speed is via CAMAC 2407 output driver module in Branch crate (Address: B4 C3 N8 A0/A1). Bits 1 and 2 of channel A0 are sent to TEM-L when in turn operates the relays and contactors to the dome motors to start movement in a CW or CCW direction respectively. Bits 1 to 8 (2 BCD decades) of channel A1 set the dome speed. These are applied to a DAC circuit mounted in another dome wall box which connect up to the TASC unit which governs the speed. Normally these bits are all set high and the dome moves at it maximum velocity.

Dome position is measured using a 10 bit T + R absolute encoder. The encoder and a line driver box are located just below the balcony floor at the east end of the walkway. A steel cover plate needs to be removed to gain access.  The signals from the line driver box  pass to a CAMAC PR2403 input register (Address: B4 C3 N10 A0)

Refer to the INT CAMAC manual for more details and TEM-L manuals for more details.

Shutter operation

The shutters can be operated from either the BRITTAIN control cabinet or from the DOME/SHUTTER control panel located about midway up in the engineering rack in the control room. The switches (some are keyswitches) on the Brittain cabinet must be put into the REMOTE position to allow shutter operation from the control room.  n.b. The shutters CANNOT be operated by computer control.

Two drive systems are incorporated on the TOP and BOTTOM shutters.

  1. Raise or lower the TOP or BOTTOM shutter using MICRO DRIVE (slow speed)
  2. Raise or lower the TOP or BOTTOM shutter using MAIN DRIVE    (normal speed)
Initially, the top shutter has to be raised in MICRO drive to clear the OVER MAIN TRAVEL limit. Once the limit has cleared (indicated by a lamp switching off on the dome/shutter control panel in the engineering rack) the top shutter can be raised using the MAIN drive. Just before reaching its fully open condition, the top shutter will operate a limit switch and stop. MICRO drive then has to be used again to move the top shutter to its fully open position thus enabling the telescope to observe at the zenith.

Closing the shutter follows a reverse procedure, but using the lower MICRO and MAIN push buttons. The same applies to the BOTTOM shutter. It must be raised or lowered in MICRO drive before the MAIN drive can be operated.

The bottom shutter is unique in that it also serves as a windshield. It consists of two sections. Once the first section raises past the dome aperture, the lower section is picked up and and they continue to move as a pair. In normal observing, the bottom shutter is seldom used. The only time being that if the telescope is observing close to the horizon, the top of the bottom shutter (when closed) could vignette the beam. In this case, the bottom shutter would be raised to allow the telescope to observe through an aperture.

n.b. Although it was originally planned to encode the dome shutters, this was never implemented. The Baldwin absolute encoders were removed from the shutter drives many years ago as were the TEM-L boards (in Dome Box 3).

Dome Slip Rings

To enable continuous dome rotation, two slip rings are used. The pickup boxes are located behind the steel covers at the top of the stairs that go to the balcony.

Dome Telemetry Equipment TEM-L

The telemetry equipment (TEM-L) provides the facility for transmitting contact states and TTL levels between the dome and the control room / CLIP centre. The system utilises 3 state signalling techniques to transfer information serially to and from the dome TEM-L wall boxes using a twin cable.

There were originally three Dome TEM-L wall boxes, but only boxes 1 and 2 are used now. n.b. Dome Box 3 contained TEM-L boards for shutter encoding, but this was never implemented. The dome wall boxes contain a strip heater operated by a thermostat. This prevents condensation forming during cold periods.

The control room (engineering) TEM-L crate is located at the bottom of the engineering rack. The CLIP centre (computer) TEM-L rack is located at the bottom of bay 6

Each  TEM-L crate comprises of  (* only fitted if needed) :

     Transmitter board (16 channels)
     Transmitter extender (to expand system to 32 channels)  *
     Receiver board (16 channels) (programmed with a unique address)
     Receiver extender (to expand system to 32 channels) *
     Multiplexer board
     Balanced line driver board

n.b. It should also be noted that some of the receiver/transmitter/extender boards I/O's are either TTL or OPTO isolated depending on the equipment to which they are attached.

To improve noise immunity; especially through the slip rings, the communication line uses a balanced pair working at approx +/- 50V relative to ground. These lines are fused on the line driver board within the TEM-L rack.

Since the various TEM-L transmitters share a common communication line, the equipment works in half duplex mode. To enable several transmitters to share the same line, a time share (multiplexer) module listens to the line and allows its transmitter module to send information only when the line is quiet. Once the transmitter has transferred all its data to the communication line, it is switched off for a period long enough to allow all the other transmitters to send their data. To ensure that the TEM-L receiver modules decode information from the appropriate transmitter, each transmitter/receiver module is set up with its own unique address which is sent as a 4 bit data block preceding the main (16/32 bits) of data.

A 16 contact input transmitter module accepts input data for transmission at the Dome wall Box 1. The data is available as 16 TTL outputs which are fed to indicator switches on the engineering control rack. Two 16 contact input transmitter modules accepts input data from the engineering rack. This data is available as 16 opto-coupled open collector outputs at the Dome wall box 2.

A full technical description of TEM-L is beyond the scope of the document, but detailed information is available in the documentation filing cabinets in the electronics workshop and in INT Technical manuals: 56 and 57

Dome TASC unit

The Dome rotation motors are connected to three drive rollers via a combination helical/wormdrive gearbox to a TASC unit which provides an infinitely variable speed or torque drive. In effect, there is no direct mechanical coupling between the motors and the dome wheels.

A poled rotor which is fixed to an output shaft is surrounded by a tube. This is driven by a constant speed AC motor. A stationary brushless coil held on the stator is fixed to an outer casting. Magnetic flux is set up when the field coil is energised generating eddy currents in the tube and causing the rotor to revolve. A tachogenerator mounted to the output shaft of one of the drives generates a voltage proportional to speed. Cooling air is provided by a constant speed fan, driven by the motor.

Speed control is achieved by comparing a reference voltage (supplied by the speed control potentiometer or via the TEM-L system) along with the feedback signal from the tachogenerator. Differences between these voltages generate more or less current to the excitation coil thereby maintaining the set speed constant. Additional facilities are provided to vary the acceleration and the torque. Refer to the TASC unit technical manuals for further information.

Last updated: Sept 2002  ejm